docs: Updates the documentation for the wepKeys field.

Updated documentation indicates that four WEP key values must be
provided. This replaces the earlier reference to "up to four." Providing
fewer than four values causes a java.lang.IllegalArgumentException.

Documentation also references the option to provide empty strings for up
to three of the values.

Test: make ds-docs

Bug: 36949880

Change-Id: I1da9f0ecfb390623f4d53ca508a554303861ed98
Exempt-From-Owner-Approval: Docs-only change